To discover more, the Franco-Italian team studied four small samples of darkened wall paint from Villa Sora in Torre del Greco (a Roman villa near Pompeii). The experiments at the ESRF needed a minute, yet intense, light beam to detect low concentrations of elements and to provide detailed chemical information.
As reported in the journal Analytical Chemistry, the experiments revealed that the chemical composition in the altered pieces was completely different from metacinnabar and that important chemical reactions had taken place in the various samples. Thus the scientists found that the cinnabar had reacted with chlorine – from some marine source, and possibly from the ‘Punic Wax’ – which led to the formation of grey chlorine-mercury compounds. In addition, a natural chemical process known as sulfation had occurred in the calcite, resulting in the development of black coating on the painting surface. The scientists looked further and investigated the cross-section of one of the samples to map the depth of alteration of the painting. They found that this darkened layer was only around 5 microns thick and that underneath the cinnabar stayed intact. So what makes the red turn into black so quickly after exposure by archaeologists? Marine Cotte, the first author of the paper said that in addition to the unstable chemical distribution of the samples ‘atmospheric conditions [sun, and possibly rain] probably play a role in this change of colours’. She added ‘Atmospheric contamination or bacterial activities can also contribute to sulfation mechanisms’.
The new research carried out at the ESRF is of major importance for future methods of preservation of Roman wall paintings at sites such as Pompeii and Herculaneum. However, it is still early days and this research is ongoing: ‘The next step is to examine more samples and not only from frescoes in the archaeological site but also from those in museums. In this way, we will be able to compare the results and better establish the causes for their degradation’, explained Marine Cotte.
This article is an extract from the full article published in World Archaeology Issue 21. Click here to subscribe